System and method for laser-based, non-evaporative repair of damage sites in the surfaces of fused silica optics

Abstract

A method for repairing a damage site on a surface of an optical material is disclosed. The method may involve focusing an Infrared (IR) laser beam having a predetermined wavelength, with a predetermined beam power, to a predetermined full width (“F/W”) 1/e 2 diameter spot on the damage site. The focused IR laser beam is maintained on the damage site for a predetermined exposure period corresponding to a predetermined acceptable level of downstream intensification. The focused IR laser beam heats the damage site to a predetermined peak temperature, which melts and reflows material at the damage site of the optical material to create a mitigated site.

What is claimed is: 
     
       1. A method of repairing a damage site on a surface of an optical material comprising:
 focusing an Infrared (IR) laser beam from a laser, of a predetermined wavelength, with a predetermined beam power, to a predetermined full width (“F/W”) 1/e 2  diameter spot size on the damage site; 
 maintaining the focused IR laser beam on the damage site for a first time duration at the 1/e 2  diameter spot size to produce a first, predetermined, continuous level of beam power, which produces a predetermined acceptable level of downstream intensification; 
 further using the focused IR laser beam at the 1/e 2  diameter spot size to heat the damage site to a predetermined peak temperature, which melts and reflows material at the damage site of the optical material to create a mitigated site; and 
 while maintaining the 1/e 2  diameter spot size constant with that used during the first time duration and during the melting and reflowinq of the material at the damage site, subsequently ramping down the beam power to a reduced, second beam power level over a second time duration, the second beam power being at least just below the glass transition temperature of the optical material. 
 
     
     
       2. The method of  claim 1 , wherein the material at the damage site of the optical material is melted and reflowed without causing evaporation of the material. 
     
     
       3. The method of  claim 1 , further comprising controlling the beam power of the IR laser beam to:
 subsequently ramp down the beam power linearly to zero over the second time duration. 
 
     
     
       4. The method of  claim 1 , further comprising controlling the beam power of the IR laser beam to:
 subsequently reduce the beam power to the second, predetermined continuous level, in step fashion, during the second time duration. 
 
     
     
       5. The method of  claim 1 , wherein the beam power is reduced from the predetermined beam power during the second time duration, to reduce stress in the optical material. 
     
     
       6. The method of  claim 1 , wherein the predetermined exposure period comprises a length of time between about 210 seconds to about 270 seconds. 
     
     
       7. The method of  claim 1 , wherein the laser beam power comprises a power within a range of about 9.75 watts to about 10.25 watts to achieve a desired irradiance level and the predetermined peak temperature. 
     
     
       8. The method of  claim 1 , wherein the IR laser beam is modulated at a frequency of about 10 kHz with an approximate 50% duty cycle. 
     
     
       9. The method of  claim 1 , wherein the optical material comprises fused silica. 
     
     
       10. The method of  claim 1 , wherein the predetermined peak temperature comprises a temperature between about 1900 K to about 2500 K. 
     
     
       11. The method of  claim 1 , wherein the temperature of the material at the damage site is reduced to just below about 1400 K, to reduce stress in the optical material. 
     
     
       12. The method of  claim 1 , wherein the operation of focusing an IR laser beam comprises focusing a quasi-continuous-wave CO 2  laser beam operating at as wavelength of about 10.6 um. 
     
     
       13. A method of repairing a damage site on a surface of an optical material comprising:
 focusing an Infrared (IR) laser beam from a laser, of a predetermined wavelength, with a first, predetermined beam power level, to a predetermined full width (“F/W”) 1/e 2  diameter spot size on the damage site; 
 maintaining the focused IR laser beam at the first predetermined beam power level, and at the 1/e 2  diameter spot size, on the damage site for a predetermined exposure period forming a first time duration; which heats the damage site to a predetermined peak temperature of between about 1900 K to about 2500 K, which thus melts and reflows material at the damage site of the optical material to create a mitigated site; and 
 while maintaining the 1/e 2  diameter spot size constant with that used during the first time duration and during the melting and reflowing of the material at the damage site, subsequently ramping down the beam power to a reduced, second beam power level over a second time duration, the second beam power being at least just below a glass transition temperature of the optical material to thus reduce stress in the optical material. 
 
     
     
       14. The method of  claim 13 , wherein the operation of maintaining the focused IR laser beam on the damage site for a predetermined exposure period comprises maintaining the focused IR laser beam on the damage site for a time period corresponding to a predetermined acceptable level of downstream intensification that is caused by the mitigated site when a subsequent laser beam is directed through the mitigated site. 
     
     
       15. The method of  claim 13 , wherein the operation of maintaining the focused IR laser beam on the damage site comprises maintaining the focused IR beam on the damage site for a time duration within a range of about 210 seconds to about 270 seconds. 
     
     
       16. The method of  claim 13 , wherein the beam power of the IR beam is controlled by:
 reducing the beam power, in a ramp fashion, from the first predetermined, beam power level applied during a first time duration, to zero over the length of the second time duration; or 
 reducing the beam power in a truncated ramp fashion from the first, predetermined level during to the reduced, second beam power level of beam power during a second time duration, where the reduced, second beam power level is at least less than the glass transition temperature of the optical material but is greater than zero. 
 
     
     
       17. The method of  claim 13 , wherein the operation of focusing the IR laser beam comprises focusing a quasi-continuous-wave CO 2  laser beam operating at a wavelength of about 10.6 um, and producing a beam diameter of no more than about 2.1 mm, and wherein the IR laser beam is modulated at a frequency of about 10 kHz with an approximate 50% duty cycle.